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How To Expose (Your Picture) Properly, For Photography Beginners: The Theory

Updated on March 30, 2011

What Is Exposure

Even though digital cameras are more common nowadays, many of the concepts of digital photography are inspired if not named after the film photography of previous years. It is useful for us to have a quick understanding of how film cameras work hence.

Film is but some sort of plastic sheet with silver salts coated on top. However, the silver coating is actually light-sensitive, and the individual grains of light-sensitive salt will chemically react to any light it receives, turning variably darker as a result as the light falls. That is basically why your film negatives look the way they do, for those who have handled film before. For colour images, the concept is similar, but there are multiple layers instead, each layer chemically manipulated to be sensitive only to a certain colour component of light e.g. red; green; blue light. Remember how white light is actually a combination of several colours i.e. your rainbow colours? Yes, that is where all your colour component light comes from to react to the specific layers.

The camera lens does the work of focusing reflected light rays (remember now, that that's how the human eye sees as well) in such a way that it all falls nicely across the film, rather than having a blurred, unfocused image. The film is safe behind a shutter which blocks out all the light from reacting with the former when you don't need it yet - you do not want to over-expose your film or the image will be too bright, or under-expose it or it will be too dim. It is only when you have framed the shot you want, then you press the shutter button to open it. That is when the magic of the light reaction occurs, and when the shutter door shuts after a while, you have your image, nicely focused and therefore, "properly exposed"!

And that is what exposure is. Photography, whether film or digital, is at its most fundamental level about exposure - by that we mean playing with the amount of light your film or light sensor receives, in the case of digital cameras which have replaced the film with the latter.

Playing With Light: Aperture, Shutter Speed, ISO

Most of us on this article probably have no idea what is happening "behind the scenes" when we take a shot.The digital camera is working quickly, sometimes within milliseconds, using its combination of processors and sensors, coupled with its understanding of human visual perception, to correctly expose the picture - that is to say, it must determine the right amount of light to enter the sensor.

The individual light-sensitive photodiodes on our digital camera sensors have replaced the single-usage silver coats of film, converting light received to electrical voltage, then digital data, to generate your final image. Colour filters on top of the sensor have also done away with the need for multiple layers of silver coats for colour imaging. Yet still, the theory and reliance on light and the need for a proper exposure is the same. As earlier mentioned, if we have too much light, our picture will come out too bright or "washed out". Too little light, it will be too dim with "shadows". How does the camera control the amount of light falling upon the sensor?

It comes down to these three variables it can play with: aperture; shutter speed and ISO.

Aperture: Look at the seven blades close in on this lens, leaving an approximate circular hole for light to pass through to the sensor.
Aperture: Look at the seven blades close in on this lens, leaving an approximate circular hole for light to pass through to the sensor. | Source

Aperture: How Wide Is Your Lens Opening

Aperture is the most straightforward and simple way to control the picture exposure. Inside the lens unit, there exists a certain mechanism that closes in or expands out, subject to the will of the operator. This mechanism, known as an iris (as I shall later explain), is usually made out of opaque, blade-shaped parts, closing in to leave an approximate circular pattern open on the lens (Refer to image). The opening or closing of this iris will directly affect how much light enters through the lens into the sensor of your camera.

In fact, the function of this is similar to how our eye pupils works. When you are in a dim-lit place, your pupils seem to dilate or expand to allow in more light, while the reverse is true under brighter lighting conditions. Internally, your iris is doing the adjustment, and the former we name the earlier-mentioned mechanism after.

The term aperture is used simply to refer to the ratio of the opening of the iris. Consequently, you may also hear it synonymously used with f-number or f-stop. This is because it is expressed as a fraction of how much of the lens is exposed to light. If the iris covers half of the area of the lens, we say that the aperture now is f/2 (said as "f-two"), just as we would say "one-over-two" for a usual fraction. If it covers three-quarters, it is f/4. If it only leaves 1/22 of the lens area open for light to pass through, it is f/22. The wider the lens opening is, the more light can pass through in a given instant - easy to see? What most amateurs might be confused with is the inverse relation of the f-number to the actual width of the opening - just remember that it is a fraction. F-big number, narrower hole. F-small number, wider hole. Also, the vocabulary is tricky. To avoid mix-ups because one could say "big aperture, small hole" and eventually confuse "aperture" with the "hole", I usually reserve the adjectives "big" and "small" when I am talking about the f-number, and "wider" and "narrower" when I wish to refer to the physical opening so that even when I say use the phrases "wide aperture" and "small aperture" interchangeably, I can very easily know which is which, because a number cannot be wide or narrow (Smiley face).

Interestingly, because of the laws of physics, a side effect comes alongside with the variable aperture. A wider aperture (see, as long as the adjective is correct, I can use "aperture" for everything!) makes it harder for the lens to make all the light rays converge properly on the sensor. Some light rays, especially those reflected from regions further away from the point of focus, cannot converge on the sensor enough to give a sharp result. Hence, the area in focus is typically smaller - the area of focus we know as depth-of-field (DOF), and hence a wider aperture leads to a more shallow DOF. Conversely, a narrower aperture leads to a better-focused image and a less shallow, or larger DOF. While not directly related to exposure, this bit of information about DOF has certain implications for applied photography.

Bear all that newly learnt knowledge about aperture in mind now as we discuss the next bit of theory - the shutter speed.

Effects of "camera shake"
Effects of "camera shake" | Source

Shutter Speed: How Long Your Shutter Stays Open

Shutter speed, luckily, is a lot easier to understand. It refers to the amount of time the shutter stays open from the moment you press the shutter button. Until there is a need for a picture to be taken, the sensor sits behind the shutter, protected by the latter against all light coming from the Universe in its dark, unlit space within the camera just like a sheet of photographic film. The moment you need to take a shot, however, the shutter opens and light floods into the sensor at once. As how I described earlier on, the light sensed is converted finally to your digital picture file from this process.

Then, the longer the shutter remains open, the more light there is coming in over time. A long shutter is not always desirable though, especially if most of the time you are shooting without a tripod. While the shutter is open, any movement of and in the scene e.g. subject movement or photographer hand shakes (we call this "camera shake"), will give you a blurred image because the changed light rays are re-projected onto the same sensor photodiodes, which "merges" them with the older light and colour information it has. In extreme cases, you get "ghosting". Hence, if we can have it, we mostly use a fast shutter speed for stationary subjects to avoid the issue of camera shake.

Aesthetically however, if there is a moving subject you are trying to capture, a longer shutter speed could be more desirable than a fast one for the "ghosting" could bring out the sense of movement in the picture. We will look into that later.

You may already realize the relation between shutter speed and aperture. The latter is akin to a "rate of light coming in" measure, and when applied to the shutter speed, gives you sort of a total control of the amount of light coming into your sensor. But that is still one more trick up our sleeve to complete the equation - the ISO.

Viewing the image at full size, can you see the random jitter, grainy bits around? The wall is otherwise supposed to be a clean, solid purple. The grainy bits are what we call camera noise, affecting image quality and making it look flat.
Viewing the image at full size, can you see the random jitter, grainy bits around? The wall is otherwise supposed to be a clean, solid purple. The grainy bits are what we call camera noise, affecting image quality and making it look flat. | Source

ISO: Sensor Sensitivity

Sure, you can now manually control how much light enters physically to your sensor via the aperture and shutter speed. What if you are shooting in a really dark place though? A dimly lit environment, or a night scene for instance, where there is simply not enough light and a longer shutter is not desirable?

Come in the ISO. ISO has to do with how sensitive the sensor is to every bit of light that strikes its photodiode. For film photography, the film is sold in different packagings or "film speed". Higher speed film rolls would have larger grains of silver on its coating than lower speed films, allowing a greater reaction under the same light - hence being more sensitive or higher speed hence of reaction. They were sold in rolls of "ISO 100"; "ISO 400"; "ISO 1600" etc. with the larger number denoting a higher speed. The "ISO" actually does not mean anything but is just a reference to the International Organization for Standardization which standardized the film speed measurements across manufacturers.

In digital sensors, the ISO reference is retained. Since there are no physical silver grains, the ISO works by varying the amplification of the electrical signals converted from the light that falls on the photodiode. The greater the ISO number, the greater the light-electrical signal, is amplified, and hence the more sensitive your sensor is to light.

ISO sounds like a good way to amplify the amount of light perceived by your sensor from whatever little light there is, thus helping you with your effective exposure level. To me, adjusting the ISO value is like a last resort, it is "artificial" rather than "natural" compared to adjusting the aperture and shutter speed. This is because there is always inherent noise (random fluctuations, not necessarily acoustic) in any electronic system and some of the light energy received is not perfectly translated to electronic signals; instead it may be lost as heat, or random signals. This is a physical law. Think of yourself turning up the radio volume - you hear louder music, but also a greater static buzz. Hence, when you are amplifying the signal received by the sensor, you are not only amplifying the light-electric signals, but also the electronic noise signals. If you look at any picture close enough, you will see these random coloured pixels scattered all over the place as a result of this unwanted noise. While they may not necessarily change your picture to something else; you can probably still identify your subject, noise will have a definite negative effect on the quality of your shot.

The amount of noise generated depends from camera to camera, sensor to sensor. This is why SLRs generally perform better under lower light. They usually have larger sensors, meaning that for the same number of photodiodes as a compact camera, the photodiodes can be made larger, maximizing the light received per diode (being more sensitive inherently without "artificial ISO amplification") and have a higher throughput of the light-electronic signal conversion.

In case you are thinking that noise is a digital-only problem, the larger silver grains on film also create a "coarser" image without the ability for finer gradations with smaller grain - it affects the finished photo in the same way as digital noise does to our pictures.

Humans 1 v AI 0

That is it for the three gateways of light - aperture; shutter speed; ISO. But first, why do we want to set all these manually?

Truth to be told, even though most digital cameras boast an automatic mode to set these variables without user intervention, the camera processor is still not the human eye. It uses algorithms and guesswork to formulate a picture it thinks is best for our visual perception. Sometimes, it fails because humankind tend to know ourselves, and how we like to see what we see, better. Other times, it is confused by objects in the scene that trap its processor in some kind of loop. It goes - is it a face? Is it a light source? It cannot tell.

Another reason is because the processors are programmed to go for the lowest common denominator i.e. the effect or finish that most people usually want. In automatic mode for instance, when capturing a wide scenic shot of mountains and forests, generally people want detail - all of the scene should be in maximal focus. The camera might auto-detect the scene as such, and adjust the aperture to achieve a maximal DOF accordingly hence. However, what if for some quirky creative reason, I want to emphasize just the mountain peak? The camera does not have a million modes for a million scenarios - that is when knowing how to override the camera defaults prove useful.

Now that you have learnt the theory, you may wish to proceed to the next part of this feature on the practical aspect of exposure.


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